This invention relates to a system and method for dispensing metered amounts of viscous liquid onto a medium.
Different types of machines are used for dispensing small metered amounts of liquid for a variety of applications. In the assembly of surface mount printed circuit boards, one application is for dispensing many small dots of adhesive liquid on a circuit board for connecting components; another is for dispensing material over an area for encapsulating chips and/or for underfilling flip chips. Such dispensing machines are expected to run continuously to achieve high throughput, and are also expected to have a high degree of repeatability, i.e., to be able to dispense dots or areas with the same size within a very small tolerance.
Some systems for dispensing dots, including earlier designs, used bursts of high pressure on a container of liquid and had a separate valve for controlling flow. This type of system was improved upon with a system that used a positive rotary displacement pump instead of bursts of high pressure. In one model manufactured by Knight Tool Co., the assignee of the present invention, and distributed by CAM/ALOT under the name Camelot(copyright), an augering screw is housed in an augering chamber and is rotated to provide a carefully metered amount of liquid. A motor is coupled to the screw with a controllable electromagnetically operated clutch. The clutch has a top plate that is continuously rotated by the motor, and a bottom plate that is rotatably coupled to the augering screw through intermediate coupling members, including a metal bellows. The liquid to be dispensed is held in a cylindrical container, and is provided to the augering chamber under constant pressure of about 10 PSI, a pressure that is considered rather low.
To dispense liquid, a controller provides to the clutch 5 a short, timed, electrical signal that induces magnetic attraction between the top and bottom plates; This attraction causes the plates to be engaged and to rotate together for a short period of time, thus causing the augering screw to rotate to dispense a small amount of liquid from the augering chamber through a nozzle. The controller also controls lead screw motors for moving the dispenser to a desired location along three mutually orthogonal axes.
Other systems that use a positive rotary displacement is pump drive the pump with a stepper motor that has to be turned on and off frequently. Such motors, however, must be sufficiently durable so that they can start and stop many times without failure.
Dispensing system of this general type can be used to 20 dispense an area of highly viscous liquid to cover the top of a semiconductor device for encapsulation, or to provide underfilling around and under a flip chip to provide thermal conductivity. Then the liquid hardens over or around the device, it packages and protects the device. To cover the area, the dispenser can be moved in a selected pattern, such as a rastering pattern or a spiral pattern.
In such dispensing systems, the accuracy of the volume dispensed is critically important. The material used for encapsulation and underfill is filled with abrasive particles and has a very high viscosity, typically from 105 to 106 centipoise. Consequently, pressure of at least about 30 to 40 PSI is provided to the container that holds the liquid to overcome friction in the container and to dispense the liquid. If the pressure is insufficient to properly feed the augering screw, cavities of air can develop in the liquid, adversely affecting accuracy (this problem is known as xe2x80x9ccavitationxe2x80x9d). Because this pressure is relatively high compared to the dot dispensing systems, however, the liquid can bleed through the augering chamber and leak through the nozzle between dispensing cycles. Such leaked liquid can have a substantial detrimental effect on the accuracy.
To prevent such bleeding and leaking, some models of dispensers have a direct drive stepper motor with a reversible drive for drawing the liquid back after a dispensing cycle is completed. Reversing a motor quickly 10 and frequently can adversely affect wear on the motor, however, and can also adversely affect accuracy.
An object of the present invention is to improve the is accuracy of a liquid dispensing system.
Another object of the present invention is to provide a reliable liquid dispensing system that does not leak between dispensing cycles.
Still another object is to provide a liquid dispensing 20 system with parts that can withstand repeated use and thus last for a long time.
Yet another object is to provide a dispenser that meets these other objects with a compact assembly.
The present invention includes a liquid dispensing 25 system and a method for dispensing while substantially preventing undesired leakage through an outlet of a nozzle between dispensing cycles. The dispenser has a cartridge with a housing that encloses an augering screw and an augering chamber. The chamber receives a liquid input, and the augering screw augers the liquid from the chanter, through an outlet in the nozzle, and onto a medium. The augering screw also serves as a valve between the augering chamber and the outlet of the nozzle. The augering screw is controllably moved so that one end moves in and out of a valve seat between the augering chamber and the outlet of the nozzle. The valve seat can be formed in the nozzle itself, or between the augering chamber and the nozzle. When the screw is in the valve seat, the seal is sufficient to substantially prevent the liquid from flowing past the seal.
The augering screw preferably has an improved design in its threading and shape, including a curved contour along the axial direction between threads. This contour reduces air spaces around the screw and increases the percentage of the area of the screw that is in contact with the liquid.
Because of the improved amount of contact area, there is less opportunity for air pockets to form around the screw, thus reducing changes in pressure that can otherwise cause leaking. To increase the amount of fluid that is dispensed per rotation, the threads of the screw are formed with a plurality of helical channels, preferably two channels 180 apart about the screw. The channels are formed so that the improved screw has double the number of threads per inch compared to known prior screws.
In one preferred embodiment, the augering screw is 20 formed integrally in a drive shaft so that the drive shaft and screw move together axially and rotatably. An annular clip is rigidly connected to the drive shaft and a piston is disposed around the drive shaft and under the clip so that it abuts the clip. The piston is biased downwardly with a spring but can receive upwardly directed gas pressure, preferably air pressure, thus causing the piston to be raised against the clip so that the screw is raised away from the valve seat. Then the air pressure stops, the screw is biased downwardly by a second spring so that it rests in the valve seat. The spring can be replaced with a gas inlet so that the piston is both raised and lowered through the use of gas pressure. A manually accessible, adjustable, threaded micrometer can be provided over the piston to control spacing between the retainer clip and the shaft. This micrometer allows the user to adjustably control the height by which the shaft and screw are raised, preferably over a range of about 0-0.1 inches.
In another embodiment, the screw includes an annular nut around its lower end. The nut and the housing around the augering chamber define an annular valve seat. The augering screw is usually biased downwardly in a position such that liquid can flow. To shut off the flow of liquid, the screw is raised. By raising the screw to prevent flow, a slight suction is created, thus further preventing leakage. The downward biasing can be achieved with a spring, or gas inputs can be provided to both raise and lower the screw.
The invention further includes a method for dispensing that includes steps of axially moving an augering screw away from a valve seat, rotating the screw to dispense liquid, and axially moving the augering screw into the valve seat to seal the liquid in the augering chamber from the nozzle. Prior to these steps, a step of adjusting the amount by which the shaft is raised can be performed. These steps of axially moving can be done by raising the screw away from the nozzle to allow flow, and lowering the screw toward the nozzle to seal the chamber, or vice versa.
In workable alternative embodiments, a liquid dispenser has a needle valve in which a vertically oriented needle is raised and lowered to form a seal with the valve seat. Extending at an acute angle relative to the vertical needle is a cartridge with an augering chamber for receiving liquid and an augering screw. In this embodiment, the augering screw meters the liquid while the needle valve is separately actuable to open and close the valve. While such a device is a workable embodiment, it is less desirable than using the screw to perform both the metering and the valving functions because it is much less compact. Another separately controllable valve can be provided, such as a rotatable ball valve having an opening that can be aligned with the passage between the augering chanter and the outlet of the nozzle.
A single augering screw serving as both a valve and an auger allows the dispensing and valving functions to be performed efficiently and compactly. By using an improved design for the augering screw, the volume of liquid that can be dispensed per revolution is increased, and leakage is further reduced because there is less change in pressure around the screw. Other features and advantages will become apparent from the following detailed description, drawings, and claims.